The use of a work tool on such steam generators, conventionally comprising, within a closed containment, at least one transverse plate, called a tube plate, which separates this containment and from which extends a plurality of parallel tubes which pass through the plate and through which flows a pressurized fluid exchanging heat by way of their wall, demands rapid, accurate and reliable positioning of the tool in relation to the axis of each tube on which work is to be carried out, from a region located on one side of the tube plate which is located within the containment and into which this tool is introduced.
In view of the conditions prevailing in the containment and particularly because of the presence of radioactive radiation, it is obvious that this positioning of the tool in relation to any of the tubes to be treated has to be carried out at a distance and by remote control, the tool being loaded on or fastened to a remote manipulator arm making it possible to displace it in the three spacial directions, especially to reset it in relation to a given reference direction which is usually formed by the axis of the tube in question. Moreover, to prevent any risk of damage to the tubes or to the tool, it is virtually indispensable that the centering of one tube in relation to another should take place without any physical contact between them.
There are already known devices of this type suitable for executing the centering of such a tool in relation to the generally vertical axis of a tube belonging particularly to a steam generator, these devices employing mechanical feelers which, precisely, necessitate direct contact between the tool and the tube plate, with the disadvantages already mentioned, which the subject of the invention proposes to avoid.
Moreover, other solutions have been considered, such as particularly in FR-A-2,628,671, which illustrates a remote-manipulation assembly ensuring the desired centering, controlled at a distance and without any physical contact between the tool and the tube. In principle, the device described in this prior patent comprises a rail-shaped radial support capable of pivoting about the center of the tube plate and extending under and parallel to this plate. A carriage designed to slide along the length of this arm comprises an axle on which a photographic camera is mounted and a second axle carrying the working and monitoring tool. The camera objective, which is directed towards the tube plate, and the axis of which is thus parallel to that of the tubes, determines the contour at the end of this tube as a result of the contrast generated by activating means for illuminating the inner zone of the tube viewed by the camera. Computation and control means connected to the camera are then operated in order to locate exactly the position of the axis of this tube, with which the axis of the camera is brought into close coincidence as a result of a suitable displacement of the carriage. Once this operation has been carried out, the carriage is displaced once again in order to center the tool in the axis of the tube by means of a second translational movement, which thereby substitutes the tool for the camera, making it possible to work inside the tube with a perfect positioning in relation to the latter.
The disadvantage of such a device is that it is necessary to employ means which are generally complex to control and normally costly, these means comprising particularly a computer and carriage-position monitoring sensors, the indications of which are managed by this computer. Furthermore, since the sight axis of the camera is different from that of the tool, a translational movement is necessary each time in order to substitute one for the other, thus adversely affecting the final accuracy of the centering obtained.